1. Field of the Invention
The present invention relates to a liquid crystal display apparatus and more particularly to a tablet integrated liquid crystal display apparatus in which a tablet input apparatus and a liquid crystal display apparatus are integrated.
2. Description of Related Art
The apparatus that a tablet input apparatus and a display apparatus are unified is known as an input apparatus to input an instruction or data to a computer. In this apparatus, the instruction or data is inputted by directly touching the tablet input apparatus.
For example, as shown in FIG. 1, a display apparatus 12 and a tablet 13 are formed independently, and they are made as a unitary apparatus. In this example, as a display apparatus, a liquid crystal display apparatus is used, and as a tablet, a resistive film type tablet is used in which a small gap is provided between two resistive film sheets connected to a power supply and current flowing when an input pen 1 is used to contact the two resistive film sheets each other is detected so that the contact position is detected.
Next, FIG. 2 is a cross sectional view illustrating the structure of the tablet integrated type liquid crystal display apparatus shown in FIG. 1. In FIG. 2, a reference numeral 15 is a tablet, 16 and 17 are a set of polarizers, 18 and 19 are an opposing glass substrate and a thin film transistor (TFT) glass substrate, and 20 is a twisted nematic type (hereinafter, to be referred to as a "TN type") liquid crystal. A reference numeral 21 is a backlight which functions as a light source.
In a conventional active matrix driving type liquid crystal display apparatus, glass substrates having the thickness of about 0.6 to 1.1 mm are used as the substrates which sandwich the liquid crystal. Therefore, there is a problem in that the conventional liquid crystal display apparatus is heavy because the thick glass substrate is used. However, it is difficult to make the thickness of the glass substrate thin in the viewpoint of mechanical strength.
On the other hand, in a case where a pen input type screen input apparatus is combined with such a liquid crystal display apparatus, when an instruction or data is inputted by contacting the input pen 14 with the screen input apparatus, there is a problem in that the thickness of the viewing-side glass substrate directs a parallax between a tip of an input pen 14 and a liquid crystal display image so that it is not easy to input the instruction or data. In order to eliminate the parallax, if film substrates having the thickness of about 0.1 mm are used, when a film is deposited on the substrate on which switching elements are formed, there is a case that the substrate bends due to stress because the substrate is thin. Also, there is another problem in that the substrate is bent on handling it so that the switching elements are damaged.
Generally, in a display apparatus using a TN liquid crystal, when the layer width of the TN liquid crystal is changed due to external force, the angle of torsion of the TN liquid crystal is changed at the portion where the external force is applied, such that the transmissivity remarkably changes. Therefore, in a case where a tablet integrated liquid crystal display apparatus is fabricated to have the structure shown in FIG. 2 using the TN liquid crystal, there is a problem in that, when the input pen 14 is pushed to the tablet 15 for input of an instruction or data, the layer width of the TN liquid crystal is changed at the portion where the input pen 14 is pushed and the peripheral portion, so that the transmissivity changes there. As a result, the display quality is degraded there.
In order to solve this problem, as shown in FIG. 3, a method is known in which a transparent glass or plastic protecting board 23 is inserted between the tablet 15 and the liquid crystal display apparatus 22 to prevent influence to the liquid crystal display apparatus 22 due to the force by the input pen 14. However, in order to prevent the layer width of the liquid crystal in the display apparatus 22 in a case of using this method, it is necessary that the protecting board 23 is not deformed due to the pushing force. For this reason, the protecting board needs to be thick and firm. As a result, there is a problem in that the tablet integrated type liquid crystal display apparatus becomes heavy and thick. Also, because the distance between the tip of the input pen 14 and a liquid crystal display image is widened to several mm or more, there is a problem of the parallax that the image is displayed on the position which is different from the tip of the input pen 14.